在工程技术中,如建筑、机械、造船、隧道等工程中,经常需要有一基准线或基准面。最初采用拉钢丝法,即把一根钢丝拉直,作为基准线。这一方法的优点是简单、直观,但准直距离短,精度不高。随着工程技术的发展,对准直距离的要求越来越长,精度要求越来越高。所以到了三十年代,开始采用光学工具建立基准线,它的优点是精度高;缺点是:操作 In engineering, such as construction, machinery, shipbuilding, tunnels and other projects, often need to have a baseline or datum. The initial use of wire drawing method, that is, straightening a wire, as the baseline. The advantage of this method is simple and intuitive, but the collimation distance is short, the accuracy is not high. With the development of engineering technology, the requirements of collimation distance are getting longer and longer, and the accuracy requirements are getting higher and higher. So by the thirties, began to use optical tools to establish a baseline, which has the advantage of high precision; disadvantages are: operation